1. Field of the Invention
The present invention relates to an image processing method and apparatus for performing shading correction.
2. Description of the Related Art
In a related art, a scanner reads a white board for proofreading before reading a document, thereby adjusting an output level of RGB color system outputted from the scanner so that the output level is 255 in white background and zero in black background. A processing just stated is referred to as shading correction. The shading correction is carried out to reduce a color misalignment generated upon conversion from the RGB color system that is employed in a scanner into the CMY color system that is employed in a printing machine such as a printer.
There exist the following techniques for reducing the color misalignment generated upon converting image data of RGB color system into image data of CMY color system.
In a technique disclosed in Japanese Unexamined Patent Publication JP-A 1-268352 (1989), a plurality of color parameters determined based on RGB signals are multiplied by conversion factors for respective colors which correspond to respective colors of Y, M and C when the image data of RGB color system is being converted into the image data of CMY color system.
Next, in a technique disclosed in Japanese Unexamined Patent Publication JP-A 6-326861 (1994), color conversion input data XYZ is converted into values of Lab, and YMC obtained by performing color conversion on the color conversion input data XYZ is also converted into values of Lab, thereafter setting a conversion parameter such that a difference ΔE between the values of Lab is minimized.
Further, in a technique disclosed in Japanese Unexamined Patent Publication JP-z 2002-218273, RGB image data is converted into Lab image data, and the Lab image data is then treated with the color correction and converted into YMC image data based on which YMCK image data is produced.
Even when the image data adjusted by the shading correction is corrected by the color conversion as in the cases of the above techniques, there remains a problem that a color misalignment may be generated in the YMC image data, thus decreasing the image reproducibility, through image processing on data outputted by the scanner, which data is out of the range adjusted by the shading correction due to electrical noise, a change of supply voltage, a fluorescent color, a whiteness degree of sheet, and the like element.